1. Field of the Invention
The present invention relates generally to steam turbines and more specifically to steam turbines of the type employing "axial entry" blades. The present invention has particular application to steam turbines employing axial entry, free-standing blades, but is not limited thereto.
2. Description of the Prior Art
Steam turbines of the type employing axial entry blades generally comprise a rotor having a plurality of generally axially extending grooves disposed therearound and a plurality of blades. Each blade has a root in registration with one of the grooves. Typically, the roots and grooves each have a generally fir tree shape. Blades employing this design may be of either the free-standing type or of the integral shroud type. Either way, a problem with this design is that the blades may "flop" during turning gear operation of the rotor, i.e., the centrifugal force is generally insufficient during turning gear operation to urge the blade roots radially outward against edges of the grooves, and therefore the root can rock circumferentially in its groove. "Flop" may cause root/groove fretting and undesired noise. In the case of integral shroud blades, "flop" may also cause wear of opposing shroud faces. Obviously, these are undesirable conditions.
Certain steam turbines manufactured by Brown Boveri Corporation are known to to implement one means as an attempt to solve this problem. That means comprises a plurality of radially oriented holes disposed in the bottom of each blade root. A coil spring is disposed in each hole and contacts the bottom of the groove with which the root registers. Apparently, it is the function of the springs to urge the roots radially outward against the edges of the groove with which they register. The problem with this solution is that it is mechanically complicated, and the radially oriented holes may weaken the blade root. Further, the amount of force that may be exerted by the coil spring is limited by the size of the hole that can be drilled in the root and by the size of the coil spring that may be placed therein.
The assignee of the present application has utilized two other methods in an attempt to solve the blade "flop" problem. The first involves cementing each blade root in its respective groove with an adhesive such as Loctite.RTM.. The second involves placing shims, such as kicker shims, between the bottom-most portion of each root and the bottom of the groove with which it registers. The shims urge the blade roots radially outward against the edges of the grooves. The problem with these methods is that turbine disassembly may be complicated and time-consuming.
It is therefore an object of the present invention to provide an apparatus and method for eliminating blade "flop" that is simple to implement, reliable, efficient and that renders turbine disassembly a simple matter.